The plot thickens Jonathan. So you installed a header exhaust and the problems began at that point? The large washers the B20 uses to bridge the separations between the intake and exhaust at four locations are critical to good clamping. If the flange thickness of the header is different from that of the aluminum manifold, you will have a problem with leaks. The WD-40 will show up a leak simply by changing the engine RPM. Whether it is rich or lean doesn't matter, as long as it confirms a leak.